Method for electrohydrogenation of benzene and substituted derivatives thereof



nited States Patent M 3,485,726 METHOD FOR ELEflTROl-IYDROGENATION 0F BENZENE AND SUBSTITUTED DERIVATIVES THEREOF Akira Misono and Tetsuo Osa, Tokyo, Japan, assiguors to Mitsubishi Chemical Industries Limited, Tokyo, Japan, 2; corporation of Japan, and Akira Misono, Tokyo,

apan No Drawing. Filed Jan. 30, 1968, Ser. No. 701,564 Claims priority, application Japan, Feb. 14, 1967, 42/8 977 Int. c1. Blllk 1/00 "US. Cl. 20459 18 Claims ABSTRACT OF THE DISCLOSURE A process for the partial hydrogenation of an aromatic compound selected from the group consisting of benzene and substituted derivatives thereof which comprises electrohydrogenating said aromatic compound by passing an electric current across an electrolytic solution containing said aromatic compound; a supporting electrolyte, which dissociates into quaternary alkyl ammonium cations; at least one member of the group consisting of water and lower aliphatic alcohols; and at least one solvent selected from the group consisting of aliphatic nitriles, aliphatic ethers and cycloaliphatic ethers, whereby a compound having a dihydrogenated benzene nucleus is predominantly obtained.

The invention relates to a method for electrohydrogenation of benzene and substituted derivatives thereof. More particularly, the invention relates to a method for subjecting benzene or substituted derivatives thereof to partial hydrogenation by means of electrolysis.

In recent years remarkable developments have been achieved in polarography. Attempts have been made to apply polarography to the electrolytic reduction of organic compounds. However, no economical process has been disclosed so far in which compounds of the benzene series are directly subjected to electrolytic reduction for the purpose of hydrogenating the benzene nuclei.

An object of the invention is to provide a method for the electrolysis of a compound of the benzene series in which said compound is subjected directly to electrolysis so that the benzene nucleus selectively undergoes partial hydrogenation.

Another object of the invention is to provide a method for the electrolysis of the benzene nucleus of benzene or substituted derivatives thereof in which said nucleus is subjected to partial hydrogenation so that benzene or a substituted derivative thereof is converted chiefly to a benzene compound having two more hydrogen atoms in the nucleus than the original one (which will hereinafter be referred to as a dihydrogenated benzene compound), especially to 1,4-dihydrobenzene or 2,5-dihydrosubstituted derivatives thereof.

A further object of the invention is to provide an electrolytic method for effecting partial hydrogenation of benzene or substituted derivatives thereof, said method having high current efiiciency.

Other objects and advantages of the invention will be apparent as the description progresses.

The above-mentioned objects are attained according to the invention by selectively subjecting benzene or substituted derivatives thereof to partial hydrogenation by means of electrolysis in an electrolytic solution containing a supporting electrolyte, which dissociates into quaternary ammonium cations; at least one member of the group consisting of water and lower aliphatic alcohols;

3,485,726 Patented Dec. 23, 1969 and a solvent selected from the group consisting of aliphatic nitriles, aliphatic ethers and cycloaliphatic ethers.

The invention will now be described in greater detail. The benezene or substituted derivative thereof employed as the raw material in the method of the invention, has a reduction potential more cathodic than 2.5 volts (vs. SCE) and is represented by the formula:

( rl dm wherein R and R are independently selected from the group consisting of lower alkyl-, lower alkoxy-, amino-, lower alkylamino-, carboxyl-, lower alkoxycarbonyl-, cyano-, phenyl-, and alkylphenyl radicals; in and n are integers from zero to 4, the sum of m and it does not exceed 2; however, when both R and R are lower alkyl radicals the sum of m and )1 may exceed 2 and range up to 4.

Examples of said raw material include benzene; alkylbenzenes such as toluene, Xylene, mesitylene and durene; alkoxybenzenes such as anisole; anilines such as aniline and toluidine; alkylaminobenzenes such as methylaminobenzene and dimethylaminobenzene; aromatic carboxylic acids such as benzoic acid, toluic acid and phthalic acid; benzene carboxylic acid alkylesters such as methyl benzoate, ethyl benzoate and methyl toluate; cyanobenzenes such as benzonitrile and tolunitrile; and diphenyls such as biphenyl and tolylbenzene.

The substance to be reduced is dissolved in an electrolytic solution. The concentration of said substance is usually more than 1% and preferably is more than 2%. Generally, the higher the concentration of said substance in the electrolytic solution the better the results obtained. If a concentration unduly lower than 1% is employed current efiiciency will be adversely affected.

The supporting electrolyte used in the electrolysis according to the invention is a salt which is capable of dissociating into quaternary alkyl ammonium cations thus providing an electric conductivity for the electrolytic solution. Examples of substances which may serve as the supporting electrolyte include organic sulphonates, organic sulphinates, halides, perchlorate and hexafluorophosphate having the formula:

in which R represents an alkyl radical having 1 to 8 carbon atoms; X represents a radical selected from the group consisting of Cl-, Br, 1", C10 PF ArSO and ArSO the Ar denoting a phenylor an alkylphenyl radical.

The stability of the quaternary ialkylammonium ions will be highest when the alkyl radical combined with its nitrogen atom, has 3 or 4 carbon atoms, in other words the propyland butyl radical are most stable. The stability of the radical is reduced more and more with the decrease or increase in carbon atoms of 3 or 4, respectively. Thus tetrapropylammonium ions and tetrabutylammonium ions are preferably employed, however other tetraalkylammonium ions may also serve. Examples of salts having such ions include 0-, m-, p-toluenesulphonate, benzene-sulphonate, and o-, m-, p-cumenesulphomate of tetramethylammonium, or tetrapropylammonium or tetrabutylammonium, or tetraethylammonium; benzenesulphinate and o-, m-, p-toluenesulphate of tetrapropyl-ammonium, or tetrabutylammonium; and chloride, bromide, and iodide of tetrapropylammonium, or tetrabutylammonium; and perchlorate, or hexafluorophosphate of tetrapropylammonium, or tetrabutylammonium.

Choice of the salt is determined by the magnitude of the reduction potential, actually used for electrolysis.

The concentration of the supporting electrolyte in an electrolytic solution cannot be specified as it varies, depending on the solubility of the supporting electrolyte and the kind of solvent employed.

According to the method of the invention, the solvent used for preparing the electrolytic solution of the abovementioned substances is selected from the group consisting of aliphatic nitriles, aliphatic ethers and cycloaliphatic ethers. These are good solvents for benzene and substituted benzene derivatives. They also dissolve protondonors such as water, lower aliphatic alcohols and the supporting electrolyte which is added to said electrolytic solution while maintaining a homogeneous liquid phase and enabling the reaction to proceed smoothly.

Examples of solvents having such properties include acetonitrile, propionitrile, butyronitrile, isobutyronitrile, allylacetonitrile, ethylene glycol diethyl ether, dimethoxy ethane, methyl Cellosolve, ethyl Cellosolve, ethylene glycol monopropyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, and dioxane.

When an electrolytic solution is prepared from the above-mentioned substances, a proton-donor comprising water, a lower aliphatic alcohol, or a mixture thereof is added to the electrolytic solution. The appropriate quantity of a proton-donor added to the electrolytic solution varies with the nature of the specific substance to be reduced and the kind of solvent employed. In the case of water, the amount of water added to the electrolytic solution usually ranges from 2 to 40% based upon total solution volume, and is preferably from about 3 to 30% by volume. With a lesser amount of water, the yield of the reaction products i.e., dihydrogenated products is reduced, whereas with a greater amount of water, dissolution of the substance to be reduced becomes difficult.

When lower aliphatic alcohols such as methanol or ethanol are employed, a larger quantity of said alcohols is required as compared with water and the amount used is usually from 5 to 40% by volume. When water and alcohol are concurrently used, the amount used usually ranges from 3 to 50% by volume.

In accordance with the invention the electrolytic cell employed in the electrolytic reduction is made of any type of material that withstands the action of the electrolytic solution. Preferably, the electrolytic cell comprises an anode chamber, a cathode chamber, and a partition wall disposed between said chambers, said wall being made of a cation exchange membrane, porous sheet, or glass filter permeable to cations.

Any metal having a more cathodic hydrogen overvoltage than tin, such as mercury, cadmium, lead, and tin, may be employed as the cathode. A platinum or carbon electrode is employed as the anode.

When an electrolytic cell comprising an anode chamber and a cathode chamber, separated from one another, is employed to carry out an electrolytic reduction in accordance with the invention, it is advantageous to employ as the anolyte an electrolyte comprising a supporting electrolyte, a member selected from the group consisting of water and lower aliphatic alcohols and a solvent, and as the catholyte an electrolyte comprising a mixture of said electrolyte and benzene or a substituted derivative thereof which is to be reduced. The electrolysis is carried out by bringing said catholyte into contact wit-h the cathode made of a metal having a more cathodic hydrogen overvoltage than tin.

While the suitable cathode potential during the electrolysis varies with the nature of the substance that is to undergo reduction, it must be more negative than the reduction potential of the said substance. However, it should be noted that if the cathode potential is too negative quaternary ammonium ions of the supporting electrolyte and solvent will decompose and adversely affect the reaction.

The reaction products resulting from the electrohydrogenation process of the invention are separated by conventional methods such as extraction and fractionation and are collected. For example, a quantity of water is added to the reaction products to form an oily phase and aqueous phase. The aqueous phase and oily phase are separated. The oily phase is then dried, subjected to distillation, and the desired end product yielded is then purified.

The reaction products were confirmed by means of infrared spectrum, nuclear magnetic resonance spectrum and gas-chromatography.

In the electrohydrogenation process of this invention, the substance undergoing reduction directly receives electrons from the cathode for reduction. Hence, the electrohydrogenation process of the invention differs from the prior art process, i.e., so-called indirect electrolysis in which an electrolyte containing a metal salt such as sodium salt is subjected to reduction to form metallic sodium, which in turn reduces a substance that is to be reduced.

The present invention is illustrated in the following examples.

EXAMPLE 1 The reaction liquor which served as the catholyte was prepared by dissolving 3.7 g. benzene and 8.0 g. tetrabutylarnmonium bromide in a mixture of 49 ml. diethylene glycol dimethyl ether and 4.5 ml. water. The solution, which served as the anolyte was prepared by dissolving 4.0 g. tetrabutylarnmonium bromide in a mixture of 25 ml. diethylene glycol dimethyl ether and 2.5 ml. water. A constant-voltage electrolysis was carried out with a 3.1 v. cathode potential.

The electrolysis cell was provided with a partition wall made of glass filter, said wall served to divide the cathode and anode chambers. A mercury electrode having a surface area of 50 cm. was employed as the cathode and a platinum electrode was used as the anode. Gas chromatographic analysis of the reaction liquor was made when the quantity of electricity passing through said liquor measured 0.063 faraday, said analysis indicated that 1.3 g. 1,4-dihydrobenzene and 0.043 g. cyclohexene were formed, 2.3 g. benzene remained unreacted and no cyclohexane was formed. This indicated the formation of 1,4- dihydrobenzene at 96% selectivity and 52% current efficiency.

Upon separation and subsequent purification, the main product 1,4-dihydrobenzene was confirmed by means of infrared spectrum and nuclear magnetic resonance spectrum.

Particulars of said gas-chromatography were as follows:

Column (Carbowax 1500, length 2 m.):

Column temp. C 45 Column inlet temp. C 100 Hydrogen carrier gas fiow velocity ml./min .30 Retention time:

Cyclohexene min 4.5

1,4-dihydrobenzene min 9.3

Benzene min 11.2

EXAMPLE 2 The reaction liquor, which served as the catholyte was prepared by dissolving 4.5 g. toluene and 15.3 g. tetrabutylammonium bromide in a liquid mixture of 49 ml. dimethoxy ethane and 4.5 ml. water. To prepare the solution which served as the anolyte 7.6 g. tetrabutylammonium bromide were dissolved in a liquid mixture of 25 ml. dimethoxy ethane and 2.5 ml. Water. A constantvoltage electrolysis was carried out with 3.1 v. cathode potential, in an electrolytic cell. The cell was provided with a partition wall made up of a cation exchange membrane disposed between a mercury cathode and a carbon anode. Gas chromatographic analysis of the reaction liquor was made when the quantity of electricity passing through the liquor measured 0.043 faraday. The analysis indicated the formation of 2,5-dihydrotoluene and cyclomonoolefines such as l-methylcyclohexene-l. The dihydrotoluene was formed with 85% selectivity and 50% current etficiency.

EXAMPLES 3-8- Various kinds of substances were subjected to reduction using the electrohydrogenation process described in Example 1. Particulars of the reaction conditions and the results obtained when 0.050 faraday of electricity were passed through the reaction liquors are listed in Table 1.

6 EXAMPLE 11 TABLE 1 Current Substance undergoing Supporting Solvent Cathode Selectivity, efiiciency, Ex. reduction electrolyte system potential, v. Chief product percent percent 3 Anisole (B11)4N-Br DEG lHzo n-.- 3. 1 zidihydromethoxy- 85 40 enzene. (BuhN-Br 1 DE GM/ 2 2 3. 1 2,5-dihydroanaline 30 20 (BuhN-Br l CH; CN/H2O 2. 7 Dihydro-benzoic acid. 20 20 u)4N-p-TSA C 3 2O- 1. 5 Dihydro-terephthalic acid- 35 27 DE 2O. 2. 7 Ethyl dihydro-benzoate... 15 13 8 Benzo-mtrrle (Et)4N-DT$A CHsCN/HzO --2. 6 Dihydrocyano-benzene 10 8 1 (BuhN-Br: tetrabutylammonium bromide. 2 DE GM: diethylene glycol dimethyl ether.

EXAMPLE 9 A catholyte and anolyte were prepared in accordance with the process of Example 2 and 7.5 g. biphenyl, g. tetraethylammonium p-toluenesulphonate and a solvent having a composition as indicated in Table 2 were subjected to constant-voltage electrolysis carried out with -2.5 v. cathode potential.

The electrolytic cell of Example 2 was employed. The results obtained after 0.050 faraday of electricity were passed through the reaction liquor are listed in Table 2. One of the two nuclei of the main product was hydrogenated. Accordingly, the selectivity was represented by the proportion (in percentage) of the main product to the biphenyl consumed.

1 (Bu)4Np'TSA: tetrabutylammonium p-toluenesulphonate. 4 (Et);N-p-TSA: tetraethylammoninm p-toluenesulphonate.

EXAMPLE 10 A reaction liquor prepared from 610 g. 1,3,5-trimethylbenzene, 7.2 g. tetrabutylammonium bromide, 49 ml. diethylene glycol dimethyl ether and 5 ml. Water served as catholyte and a solution prepared from 3.6 g. tetrabutylammonium bromide, 24 ml. diethylene glycol dimethyl ether and 3 ml. water served as anolyte. A constantvoltage electrolysis was carried out with 3.0 v. cathode potential.

Gas chromatographic analysis of the reaction liquor was made upon consumption of 0.049 faraday. The analysis indicated the formation of 1.1 g. of 1,4-dihydro- 1,3,5-trimethylbenzene, traces of dihydrogenated compound of unknown constitution and no hexahydrogenated compound. Thus, the selectivity of the 1,4-dihydrogenated compound was calculated at with 36.5% current efficiency. Upon separation and subsequent purification, the 1,4-dihydrogenated compound was confirmed by means of infrared spectrum and nuclear magnetic resonance spectrum.

Particulars of said gas-chromatography were as follows:

Column (Carbowax 1500, length 2 m.):

Column temp. C Column inlet temp. C 210 Hydrogen carrier gas flow velocity m1./min 30 Retention time:

1,4-dihydrogenated compound min 3.8 Dihydrogenated compound of unknown constitution min 6.0

Mesitylene (starting material) min 7.6

Column (Silicon oil, length 2 m.):

Column temp. C Column Inlet temp. C 200 Helium carrier gas flow velocity ml./min 50 Retention time:

Dihydrogenated compound min 13 Durain (starting material) min 15 EXAMPLE 12 A reaction liquor prepared from 5.4 g. p-Xylene, 8.0 g. tetrabutylammonium bromide, 49 ml. diethylene glycol dimethyl ether and 4.5 ml. water served as the catholyte and a solution prepared from 4.0 g. tetrabutylammonium bromide, 25 ml. diethylene glycol dimethyl ether and 2.5 ml. water served as the anolyte. A constant-voltage electrolysis was carried out with 3.1 v. cathode potential. Gas chromatographic analysis of the reaction liquor was made upon consumption of 0.0485 faraday, the analysis indicated the formation of 1.03 g. of 2,5-dihydro-p-xylene and a small amount of monoenes. Thus, the current efficiency was calculated at 39.5% for dihydroxylene and at 4.7% for monoenes, giving 94.5% selectivity of said dihydroxylene.

Particulars of the gas-chromatography were as follows:

Column (Carbowax 1500, length 2 m.):

Column temp. C 111 Column inlet temp. C 200 Nitrogen carrier gas flow velocity ml./min 50 Retention time:

2,5-dihydrogenated compound min 4.7

Xylene min 5.7

7 EXAMPLE 13 A constant-voltage electrolysis was carried out in accordance with the process described in Example 12, with the exception that in the preparation of the catholyte 5.3 g. m-xylene was employed in place of 5.4 g. p-xylene. Upon consumption of 0.0484 faraday, the reaction liquor was analysed by gas-chromatography as described in Example 12. The analysis indicated the formation of 0.915 g. of 2,5-dihydro-m-xylene and a small amount of monoenes. Thus, the current efliciency was calculated at 35.0% for dihydro-xylene and 3.1% for monoenes, giving 95.8% selectivity of said dihydro-xylene.

What we claim is:

1. A process for the partial hydrogenation of an aromatic compound selected from the group consisting of benzene and substituted derivatives thereof which comprises electrohydrogenating said aromatic compound by passing an electric current across an electrolytic solution containing said aromatic compound, a supporting electrolyte, which dissociates into quaternary alkyl ammonium ions as cations, at least one member of the group consisting of Water and lower aliphatic alcohols and at least one solvent selected from the group consisting of aliphatic nitriles, aliphatic ethers and cycloaliphatic others.

2. A process according to claim 1 wherein said electrohydrogenation is carried out by bringing said aromatic compound into contact with a cathode having a more cathodic hydrogen overvoltage than that of tin while impressing said cathode with a voltage which is more cathodic than the reduction potential of said aromatic compound.

3. A process according to claim 1 wherein said substituted derivative of benzene is a compound having at least one radical selected from the group consisting of a lower alkyl-, lower alkoxy-, amino-, lower alkylamino-, carboxyl-, lower alkoxycarbonyl-, cyano-, phenyl and alkylphenyl radical.

4. A process according to claim 1 wherein said aromatic compound has a reduction potential more cathodic than 2.5 volts (vs. SCE) and the following formula:

in which R and R are independently selected from the group consisting of a lower alkyl-, a lower alkoxy-, an amino-, a lower alkylamino-, a carboxyl-, a lower alkoxycarbonyl-, a cyano-, a phenyland an alkylphenyl radical, and m and n are integers from 0 to 4 and the sum thereof does not exceed 2, whereas in the event that both R and R are lower alkyl radicals said sum may exceed 2 up to 4.

5. A process according to claim 1 wherein benzene is employed as said aromatic compound to obtain 1,4- dihydrobenzene as main product.

6. A process according to claim 1 wherein said aromatic compound is selected from the group consisting of toluene and xylenes to obtain 2,5-dihydrogenated compound thereof as main product.

7. A process according to claim 1 wherein said aliphatic nitrile is a member of the group consisting of acetonitrile, propionitrile, butyronitrile, isobutyronitrile, and allylacetonitrile.

8. A process according to claim 1 wherein said aliphatic ether is a member of the group consisting of ethylene glycol diethyl ether, dimethoxy ethane, methyl Cellosolve, ethyl Cellosolve, ethylene glycol monopropyl ether, diethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and triethylene glycol diethyl ether.

9. A process according to claim 1 wherein said cycloaliphatic ether is dioxane.

10. A process according to claim 1 wherein said supporting electrolyte is a compound represented by the following formula: R N+X in which R is an alkyl radical having 1 to 8 carbon atoms and X* is an anion.

11. A process according to claim 10 wherein X is a member of the group consisting of Cl, Br-, I". ArSO and ArSO the Ar being a phenyl or an alkylphenyl radical.

12. A process according to claim 10 wherein R is an alkyl radical having 3 to 4 carbon atoms.

13. A process according to claim 1 wherein said electrolytic solution contains 2 to 40% by volume of water.

14. A process according to claim 1 wherein said electrolytic solution contains 4 to 40% by volume of a lower aliphatic alcohol selected from the group consisting of methyl alcohol and ethyl alcohol.

15. A process according to claim 1 wherein said electrohydrogenation of the aromatic compound is carried out in an electrolytic cell comprising an anode compartment, acathode compartment provided with a cathode having a more cathodic hydrogen overvoltage than that of tin and a cation permeable partition wall arranged betwee said compartments.

16. A process according to claim 15 wherein said cathode is mercury.

17. A process according to claim 15 wherein said partition wall is made of any one selected from the group consisting of a cation exchange resin membrane, porous sheet, and glass filter.

18. A process according to claim 15 wherein said aromatic compound is charged into the cathode compartment.

References Cited UNITED STATES PATENTS 3,361,653 1/1968 Miller 20459 HOWARD S. WILLIAMS, Primary Examiner U.S. Cl. X.R. 204-73 

